1. Field of the Invention
This invention relates to FLADE aircraft gas turbine engines and, more particularly, to such engines with fixed geometry inlet ducts.
2. Description of Related Art
High speed aircraft such as those designed to operate in the Mach 2-2.5 range typically require variable geometry inlet ducts to supply air to gas turbine engines that power the aircraft. Jet aircraft engines which are designed to operate at speeds ranging from take-off, through subsonic and transonic, and into the supersonic, require complex air inlet configurations in order to operate efficiently throughout the entire operating range. At lower subsonic speeds, particularly at take-off, it is desirable to allow the engine maximum access to air, since at these lower speeds there is no substantial “ramming” effect produced, whereby air is literally forced into the engine.
The amount of air that can be used by the engine is limited by the an inlet throat area, the throat being the point along the length of the inlet duct at which the airflow passageway is most constricted. In general, the larger the area of the inlet throat, the greater the amount of air the engine can ingest. In addition, at take-off, it is desirable to provide auxiliary airflow passageways in the inlet configuration, which have the effect of slowing the average speed of the airflow through all passageways of the inlet and, thus, preventing “sharp lip” losses (caused by separation of the airflow around the cowl lip) and “choking” of the inlet (the condition wherein the airflow through the inlet throat is sonic, i.e., Mach number=1.0, resulting in large losses in the diffusion process up to the engine inlet).
As the aircraft reaches transonic speeds, the airflow demands of the engine may also supersede the efficient supplying ability of the inlet duct because the inlet duct throat becomes choked. In the past, variable geometry inlet ducts have been designed to enlarge the minimum total cross-sectional area of the inlet duct, such as by providing auxiliary airflow passageways to the engine, in order to satisfy engine transonic airflow demand. It is well known in the art that efficient supersonic operation of the engine requires that the inlet be “started,” i.e., that the internal airflow in the inlet duct be changed from subsonic to supersonic across a shock in the inlet duct, as the aircraft speed increases to supersonic speeds.
Efficient inlet ducts which operate in a mixed-compression mode are designed to control the position of the shock in the inlet during supersonic operation so that it remains substantially at the throat to minimize pressure losses across the shock and is normal to the airflow in the duct at. See U.S. Pat. No. 4,007,891 to Sorensen for background information on supersonic inlet for jet aircraft engines. U.S. Pat. No. 4,463,772 to Ball discloses a flush inlet and inlet air passage for supersonic aircraft including provisions for efficiently decelerating a supersonic airstream entering the inlet and converting such airstream to subsonic airflow within the inlet air passage prior to introduction into the aircraft's jet engine. A two-dimensional inlet and convergent/divergent inlet air passage reduces supersonic airflow entering the inlet to transonic or subsonic velocity the airflow transits the convergent portion of the inlet passage. The subsonic or transonic flow is further decelerated as it transits the divergent portion of the inlet passage prior to introduction into the aircraft's gas turbine engine. Various apparatuses are incorporated for inflight adjustment of the inlet passage boundary walls and for removing excess low energy boundary layer air from the inlet passage surfaces.
The disadvantages of variable inlet ducts and aircraft incorporating them is their complexity and their substantial weight, which can account for a considerable portion of the total engine and aircraft weight. The complexity of these systems is increased by the fact that many aircraft being designed today incorporate engines and inlet ducts completely mounted within the aircraft's fuselage or body. Thus, it is highly desirable to have a gas turbine engine with a fixed geometry inlet duct that avoids the cost, weight, and complexity of variable geometry inlet ducts. Furthermore, it is desirable to have such an engine and fixed geometry inlet duct that can operate at full and part power conditions including take-off, landing, and cruise and that can operate and cruise efficiently over a range of supersonic flight conditions such as in a range of about between Mach 2 to 2.5.